The present invention relates to a photoconductive infrared detector, and more particularly to a highly sensitive photoconductive infrared detector with an increased electrical resistance and without substantive loss of average life time of excess minority carriers having been generated in a semiconductor crystal for detecting infrared.
The basic photoconductive infrared detector has a structure as illustrated in FIG. 1. A semiconductor crystal infrared ray detecting portion 1 is provided over a substrate 2. A plus electrode 5 and a ground electrode 6 are provided at opposite ends of the semiconductor crystal infrared detecting portion 1. The plus electrode 5 and the ground electrode 6 are distanced from each other in a lateral and longitudinal direction. A pair of top and, bottom passivation films 4 are provided on top and bottom surfaces of the semiconductor crystal infrared detecting portion 1. An adhesive is applied on a bottom surface of the bottom passivation film 4 so that the bottom passivation film 4 underlying the semiconductor crystal infrared ray detecting portion 1 is adhered on the substrate 2.
If the infrared is made incident into the semiconductor crystal infrared detecting portion 1, then excess minority carriers are generated in the semiconductor crystal infrared detecting portion 1. Those generated excess minority carriers varies an electrical resistance of the semiconductor crystal infrared detecting portion 1. The plus electrode 5 and the ground electrode 6 are biased to cause a bias current to flow from the plus electrode 5 toward the ground electrode 6. The variation in resistance of the semiconductor crystal infrared detecting portion 1 by generation of the excess minority carriers causes variation of the bias current flowing through the semiconductor crystal infrared detecting portion 1. It is therefore possible to detect the variation in electrical resistance of the semiconductor crystal infrared detecting portion 1 by detecting variation of the bias current. The variation in electrical resistance of the semiconductor crystal infrared detecting portion 1 depends upon the amount of the generated excess minority carriers. The amount of the generated excess minority carriers depends upon the intensity of the infrared having been made incident into the semiconductor crystal infrared detecting portion 1. Namely, any variation in the bias current flowing through the semiconductor crystal infrared detecting portion 1 means or teaches that any infrared has been made incident into the semiconductor crystal infrared detecting portion 1 whereby the excess minority carriers are generated in the semiconductor crystal infrared detecting portion 1 and flow through the same toward the ground electrode 6, wherein minority carriers are holes. The excess minority carriers having been generated will be recombined with electrons the average life time in the average. The average life time of the excess minority carriers depends upon the surface state and property of the semiconductor crystal infrared detecting portion 1. It is generally said that if the average life time of the excess minority carriers is long, then a variation in electrical resistance of the semiconductor crystal infrared detecting portion 1 caused by the incidence of the infrared is large. The large variation in electrical resistance of the semiconductor crystal infrared detecting portion 1 means that the photoconductive infrared detector is highly sensitive to the incident of the infrared. Namely, it is desired that the average life time of the excess minority carriers is as long as possible to ensure a possible high sensitivity to the infrared. In order to obtain a possibly long average life time of the excess minority carriers generated in the semiconductor crystal infrared detecting portion 1, it is required to prevent the recombination of the excess minority carriers or holes with electrons. In order to prevent the recombination of the excess minority carriers with electrons, the above top and bottom passivation films 4 are effective.
The above conventional infrared detector structure are, however, engaged with the following problems. In order to prevent the recombination of the excess minority carriers, the semiconductor crystal infrared detecting portion 1 are coated with the passivation films 4 but only the top and bottom surfaces thereof, whilst opposite side walls and the opposite ends of the semiconductor crystal infrared detecting portion 1 are not coated by the passivation films 4, wherein the opposite side walls are distanced from each other in a horizontal direction vertical to the longitudinal direction. The recombination velocity of the excess minority carriers is extremely large in the vicinity of the opposite side walls and the opposite ends of the semiconductor crystal infrared detecting portion 1. Namely, the excess minority carriers are immediately recombined and disappeared in the vicinity of the opposite side walls and the opposite ends of the semiconductor crystal infrared ray detecting portion 1. These phenomenon teach that the average life time of the excess minority carriers are short and thus the above conventional infrared ray detector has a deteriorated sensitivity to the infrared ray.
In order to settle the above problem with deterioration of the sensitivity to the infrared, it was proposed to other infrared detector which has a structure as illustrated in FIGS. 2A and 2B. This infrared detector is disclosed in the Japanese laid-open patent application No. 63-11821. A semiconductor crystal infrared ray detecting portion 1 is provided over a substrate 2. A plus electrode 5 and a ground electrode 6 are provided at opposite ends of the semiconductor crystal infrared detecting portion 1. The plus electrode 5 and the ground electrode 6 are distanced from each other in a lateral and longitudinal direction. A pair of top and bottom passivation films 4 are provided on top and bottom surfaces of the semiconductor crystal infrared detecting portion 1. An adhesion is applied on a bottom surface of the bottom passivation film 4 so that the bottom passivation film 4 underlying the semiconductor crystal infrared detecting portion 1 is adhered on the substrate 2. Infrared ZnS insulation film 7 transparent to the infrared is provided which extends over the top portions of the plus electrode 5 and the ground electrode 6 as well as over the top passivation film 4. Infrared shielding mask 8 is provided over the infrared ZnS insulation film 7 for shielding the infrared. The infrared shielding mask 8 is square-shaped. The infrared shielding mask 8 has a window 9 which is also square-shaped at the center position so as to allow the infrared ray to be incident through the window 9 and the infrared ZnS insulation film 7 into the center portion of the semiconductor crystal infrared detecting portion 1. The window 9 of the infrared shielding mask 8 has a sufficiently small area as compared to an area of the semiconductor crystal infrared detecting portion 1. The semiconductor crystal infrared ray detecting portion 1 is made of HgcdTe. A distance between the edge of the window 9 of the infrared shielding mask 8 and the side wall of the semiconductor crystal infrared detecting portion 1 is set larger than a diffusion length, for example, 20 micrometers, of carriers or excess minority carriers generated so as to suppress a recombination of the generated excess minority carriers in the vicinity of side walls of the semiconductor crystal infrared detecting portion 1. This contributes to suppress a remarkable loss of the average life time of the excess minority carriers generated in the semiconductor crystal infrared detecting portion 1 thereby allowing an improvement in sensitivity of the infrared detector.
The sensitivity of the photoconductive infrared detector depends not only upon the above average life time of the excess minority carriers generated in the semiconductor crystal infrared detecting portion 1 but also upon a resistance of the semiconductor crystal infrared ray detecting portion 1. If the resistance of the semiconductor crystal infrared detecting portion 1 is large, then the variation in resistance of the semiconductor crystal infrared detecting portion 1 due to generation of excess minority carriers therein is also large. If the variation in resistance of the semiconductor crystal infrared detecting portion 1 due to generation of excess minority carriers therein is large, this means that the sensitivity of the photoconductive infrared detector is also high.
The infrared ray is made incident through the window 9 of the infrared shielding mask 8 and the infrared transparent ZnS insulation film 7 into the center portion of the semiconductor crystal infrared detecting portion 1 whereby excess minority carriers are generated in the center area of the semiconductor crystal infrared detecting portion 1 and are drifted through the semiconductor crystal infrared ray detecting portion 1 to the ground electrode 6 Namely, the generated excess minority carriers flow through the half portion of the semiconductor crystal infrared detecting portion 1 in the ground electrode side, for which reason the electrical resistance to the excess minority carriers is substantially determined by the half portion of the semiconductor crystal infrared detecting portion 1 in the ground electrode side. The substantial electrical resistance of the semiconductor crystal infrared detecting portion 1 is determined between the center area of the semiconductor crystal infrared detecting portion 1 and the ground electrode 6. As well illustrated in FIG. 2A, the semiconductor crystal infrared detecting portion 1 has a wide width which remains unchanged between the plus electrode 5 and the ground electrode 6. Namely, the wide width of the semiconductor crystal infrared ray detecting portion 1 in the half area in the ground electrode 6 provides a low resistance which causes a deterioration in the sensitivity of the above conventional photoconductive infrared detector. Namely, it is difficult for the above conventional photoconductive infrared detector to ensure the required high sensitivity.
In the above circumstances, it is required to develop a novel highly sensitive photoconductive infrared detector having a high electrical resistance and ensuring a long average life time of excess minority carriers generated in a semiconductor crystal infrared detecting portion provided in the photoconductive infrared detector.